A Two-Color Heterodyne Interferometry Sensor for Direct, High-Speed Electron Density Measurements in NASA EAST Facility

Planetary entry and atmospheric reentry involve very high velocities and a significant source of vehicle heating is the radiation from the high temperature, bow shock-heated plasmas formed in front of the vehicle. The radiative environment corresponding to different planetary entry conditions is simulated in the NASA Ames Electric Arc Shock Tube (EAST) facility, and there is a need for developing nonintrusive, accurate, spatially and temporally resolved diagnostic of electron number density to help improve the physics of radiative transport prediction models. The objective of the Phase II effort is the refinement and delivery of a high-speed electron number density sensor using two-color heterodyne interferometry (TCHI) for the EAST facility. Specifically, we will build on the promising initial results from Phase I and develop a compact, robust and validated TCHI sensor integrated with EAST. Based on our initial data and additional opportunities to refine the sensor, we anticipate that electron density measurements below 1012 cm-3 with spatial resolutions of 1 mm are feasible at 1 MHz rate. Further optimize and validate the TCHI system performance using surrogate plasmas. Build a more compact and robust TCHI transmitter and detector platforms, including an all-fiber detection system. Compare the free-space and fiber detection implementations of TCHI in a shock tube test. Build a laser schlieren system for accurate detection of the shock front arrival to serve as a trigger source for the TCHI sensor. Perform final integration, testing, and system performance checks at the NASA EAST facility. The Phase II deliverables will be progress reports, a final report, two instruments a laser-schlieren system for shock front detection and the TCHI sensor, validated with measurements in the EAST facility along with their user manuals. Planetary entry and atmospheric reentry involve very high velocities and a significant source of vehicle heating is the radiation from the high temperature, bow shock-heated plasmas formed in front of the vehicle. The radiative environment corresponding to different planetary entry conditions is simulated in the NASA Ames Electric Arc Shock Tube (EAST) facility, and is used to obtain shock radiation data for the validation of phenomenological models for nonequilibrium radiation transport and improved understanding of the flow physics. There is a need for developing nonintrusive, accurate, spatially and temporally resolved diagnostic of electron number density to help improve the physics of radiative transport prediction models. Following our successful Phase I effort, we plan to further develop a high-speed, spatially resolved, two-color heterodyne interferometry (TCHI) diagnostic during Phase II for integration into the EAST facility and perform validation tests to confirm the capability to provide high precision electron number density measurements at a very high sampling rate The objective of the Phase II effort is the refinement and delivery of a high-speed electron number density sensor using two-color heterodyne interferometry (TCHI) for the EAST facility. Specifically, we will build on the promising initial results from Phase I and develop a compact, robust and validated TCHI sensor integrated with EAST. Based on our initial data and additional opportunities to refine the sensor, we anticipate that electron density measurements below 1012 cm-3 with spatial resolutions of < 1 mm are feasible at 1 MHz rate, while acquisition rates above 50 MHz will be possible at a reduced noise floor of 5 × 1013 cm-3. Further optimize and validate the TCHI system performance using surrogate plasmas. Build a more compact and robust TCHI transmitter and detector platforms, including an all-fiber detection system. Compare the free-space and fiber detection implementations of TCHI in a shock tube test. Build a laser schlieren system for accurate detection of the shock front arrival to serve as a trigger source for the TCHI sensor. Perform final integration, testing, and system performance checks at the NASA EAST facility. The Phase II deliverables will be progress reports, a final report, two instruments – a laser-schlieren system for shock front detection and the TCHI sensor, validated with measurements in the EAST facility along with their user manuals.
Benefits: The TCHI diagnostic is needed to aid tests at NASA EAST and arc jet facilities, hypersonic wind and shock tunnels. Non-intrusive measurements of electron density at high speeds and with fine spatial resolution are required for validating computational fluid dynamic modeling and simulation codes that incorporate real-gas kinetic and transport models used to predict planetary entry and earth reentry radiative heating. The diagnostics can serve as tools in high enthalpy flows that focus on testing the integrity of thermal protection systems. The capability to measure spatially and temporally resolved electron density in high enthalpy flows will be attractive to companies such as Lockheed Martin, Boeing, etc., that develop heat shields and thermal protection systems for hypersonic vehicles, and private space industries such as SpaceX, Blue Origin, etc., that develop space launch systems for low earth orbit and planetary exploration.